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Protective current requirements in CP Design
Assumptions of protective current requirements and bare metal areas. To obtain a starting point, certain general assumptions have been found helpful:
For bare metal in the ground, a current of 11 to 22 mA/m2 of bare metal surface has been found adequate, except under extreme or unusual conditions. This value must then be modified to suit the particular conditions.
For coated pipe, the current required is difficult to estimate without field tests. The primary reason is the unknown condition of the protective coat which can vary from nearly 0% to 98% coverage. For a fairly new protective coat properly applied,. assume 2 percent bare and 22 mA/m2 for use in tentative calculations. Field test may show that this figure should be modified.
Bare pipelines can usually be protected by 11 to 22 mA/m2. This is seldom justifiable economically for extensive or long lines, however, and the necessary protection is usually afforded by the application of cathodic protection to localized areas called "hot spots."